A photopolymerizable composition contains principally a photopolymerization initiator and an addition-polymerizable compound having two or more ethylenic unsaturated bonds in a molecule (which is hereinafter called "a polyfunctional monomer"), which is hardened by the irradiation of light to cause a change of tackiness and solubility in a solvent. These properties are widely used for photography, printing, metal surface processing, ink and so on. The idea and the application thereof are widely documented, and are described, for example, in J. Kosar, Liqht Sensitive Systems, J. Wiley & Sons, New York, 1965, pp. 158-193; and K. I. Jacobson and R. E. Jacobson, Imaging Systems, J. Wiley & Sons, New York, 1976, pp. 181-222.
Also, as methods of forming images by utilizing photopolymerizable compositions, image-forming systems utilizing photosensitive microcapsules, including systems wherein the photopolymerizable compositions are contained in capsules have been proposed. For example, JP-A-57-124343, JP-A-57-179836, and JP-A-57-197538 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") disclose a method of forming dye-images comprising image-wise exposing to light a color sheet coated with microcapsules containing a dye and a photopolymerizable composition comprising a vinyl compound and a photopolymerization initiator, and placing the exposed color sheet against an image receiving sheet and applying pressure on the two sheets over the whole surface thereof to rupture the microcapsules and release dye for transfer.
Recently, attempts have been made to use these photopolymerizable compositions spectrally sensitized to the region of visible rays for the formation of digital images using a laser as a light source, and for application to full-color photosensitive materials.
For example, Nippon Shashin Gakkai-shi, Vol. 49, No. 3, p. 230 (1986) and Kinoh Zairyo (which means "Functional Materials"), September 1983, pp. 48-60 disclose a method of the spectral sensitization of photopolymerizable compositions to be used for laser-recording. Also, JP-A-59-189340 discloses a method of using organic dyes for the spectral sensitization of organic peroxide initiators. EP-Al-223587 discloses organic boron anionic salts of organic cationic dyes to be used as novel photopolymerization initiators.
Particularly, the method of using the organic boron compound anionic salts of organic cationic dye compounds as photosensitive initiators is very useful in view of the possibility of preparing photopolymerizable compositions sensitive to light in any wavelength because of the wide range of choice of cationic dye compounds.
Certainly, photopolymerizable compositions having sensitivity to visible rays could be produced by these methods, but it has been found that the resulting photosensitivity thereof was not satisfactory.